Understanding and improving axial detection in optical tweezers based on the interference of forward- and backward- scattered light

TL;DR

This paper develops a comprehensive model for axial detection in optical tweezers that accounts for both forward and backward scattering, improving accuracy in particle position measurement under various conditions.

Contribution

It introduces a new model explicitly including backward scattering effects, validated experimentally, enhancing the understanding and accuracy of axial detection in optical tweezers.

Findings

The model explains standing-wave responses in optical tweezers.

Experimental validation confirms the model's accuracy.

Improved estimation of trap stiffness and particle diffusion.

Abstract

Fast and accurate 3D position detection in optical tweezers (OT) is essential for quantitatively monitoring subtle variations in the mechanical properties of microscopic systems ranging from biomolecules to cells and colloids. Because standard OT configurations do not provide direct access to the axial position, axial detection typically relies on temporal fluctuations in forward-scattered optical power to infer the position of the particle. This approach generally assumes a linear-response regime in which the signal arises from the interference between the forward scattered and the nonscattered optical fields; however, under certain conditions, the backward-scattered contribution becomes non-negligible, leading to deviations from the linear response. Here, we present a simple yet comprehensive model for axial detection in standard OT while explicitly accounting for the backward-scattered field. Together with experimental validation, this framework neatly explains the standing-wave response observed when the backward-scattered field interferes with the nonscattered and the forward-scattered components, enabling accurate estimation of trap stiffness and particle diffusion under more general conditions. This work deepens our understanding of the phenomenology observed in real optical-tweezers measurements and extends their capabilities to conditions where standard approaches fail.